Electrical systems

ABSTRACT

Electrical systems for connecting rotary electric machines to dc networks operating at different voltages V and W where V&gt;W are provided, along with gas turbine engine arrangements incorporating such systems.

CROSS-REFERENCE TO RELATED APPLICATIONS

This specification is based upon and claims the benefit of priority fromUnited Kingdom patent application number 1917888.8, filed on 6 Dec.2019, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to an electrical system for connecting anelectric machine to dc networks operating at different voltages.

BACKGROUND

In aerospace, the more electric engine (MEE) and more electric aircraft(MEA) concepts are seen as being increasingly attractive due to theirpotential to reduce fuel consumption. For example, one known aircraftconfiguration includes electric machines in its engines which areoperable as both motors and generators. This facilitates both generationof electrical power during flight and starting of the engine, allowingremoval of the air-turbine starter and attendant bleed air ducting. Oneengine configuration for this known aircraft includes such electricmachines coupled to the high-pressure spool of a twin-spool turbofan.Another includes such electric machines coupled to theintermediate-pressure spool of a triple-spool turbofan.

Analysis has shown that further reduction in fuel consumption may beachieved by generating electrical power by the low-pressure spool. Inconjunction with this, it has also been shown that transfer of powerbetween the low- and high-pressure spools also improves fuel efficiencyat a number of phases in the operational envelope. In an example,approximately 1 megawatt of power may be produced from the low-pressureshaft, with 400 kilowatts being transferred to the high-pressure spooland the remaining power supplied to the airframe.

It is also contemplated that future airframe designs may incorporatefuselage boundary layer ingestion systems to reduce wake drag, furtherreducing fuel consumption. Most practical proposals are based ontube-and-wing twinjets, in which the underwing turbofan engines alsooperate as turboelectric generators for supplying power to anelectrically-driven boundary layer ingestion fan at the tail of theaircraft. Such aft-mounted fans may command in excess of 2 megawatts tooperate effectively. Thus, each turbofan engine may be required toproduce an additional 1 megawatt of electrical power for the airframe.

In such scenarios, therefore, medium voltage (as defined by IEC60038:2009, i.e. 1 kilovolt ac or greater) electrical systems will berequired in order to maintain acceptable current ratings, as too high acurrent has a detrimental effect in terms of Joule losses and conductorweight. However, it is desirable to reduce the number of systemsoperating at medium voltage due to the risk of arcing and corona ataltitude, which can be achieved by adopting lower voltage ratings forthe lower powered systems.

In this specification, the following mathematical notation is assignedto objects for the purposes of clarity and conciseness. A collection ofobjects having an ordered relationship therebetween may be representedas a sequence of members. For a finite sequence σ of length N, eachmember has an associated index n identifying its position in thesequence.

As used herein, the length of a sequence is denoted by an uppercaseletter, with the variable representing the indices of the members of thesequence denoted by the equivalent lowercase letter. In this way it ispossible herein to concisely define properties of each member of asequence.

For example, let a sequence α have length N=8 such that the members eachhave an associated index n. It is possible to state that for all n≡0(mod 2), the nth member has property X. In this way a particularproperty is defined over the even-indexed members.

This is in contrast to having to set out that each of the second member,the fourth member, the sixth member, and the eighth member has propertyX.

Thus it may be seen that great improvements in conciseness are possiblefor sequences of large length with properties shared over amathematically-defined sub-sequence.

Furthermore, say we wish to describe links between the members of amultiplicity of sequences. For example, let a sequence β have a lengthP=3, and let a sequence γ also have a length P=3. The members of bothsequences therefore have an index p associated therewith.

Given these definitions, we may concisely state that for all p=(1 . . .P), the pth member of sequence β is linked to the pth member of sequenceγ.

This is in contrast to having to set out that the first member ofsequence β is linked to the first member of sequence γ, the secondmember of sequence β is linked to the second member of set γ, and thethird member of sequence β is linked to the third member of sequence γ.

Whilst the statements are equivalent, it will be appreciated that byusing the notation explained heretofore significant improvements inconciseness are achieved.

SUMMARY

The invention is directed to electrical systems for connecting electricmachines to dc networks operating at different voltages V and W, alongwith gas turbine engine arrangements incorporating such systems

In an aspect, there is provided an electrical system of the aforesaidtype, where V>W, and the electric machine has N≥2 independent phaseseach having a respective index n=(1, . . . , N), the electrical systemcomprising:

a first set of N ac-dc converter circuits connected in a modularmultilevel configuration, each ac-dc converter circuit having arespective index n=(1, . . . , N) and an ac interface for connectionwith a corresponding nth phase of the electric machine, and in which themodular multilevel configuration has P=N+1 dc outputs each having arespective index p=(1, . . . , P) wherein the potential differencebetween the pth output and the (p+q)th output is qV/N, where q=(0, . . ., P−p);

a set of N dc-dc converter circuits each having a respective index n=(1,. . . , N) and being configured to convert dc power between a voltageV/N at a first dc interface and a voltage W at a second dc interface,wherein, for all n, a first dc interface of the nth dc-dc convertercircuit is connected with the p=nth and p=(n+1)th dc outputs of themodular multilevel configuration.

In an embodiment, 0.5 W≤V/N≤2 W.

In an embodiment, 0.8 W≤V/N≤1.3 W.

In an embodiment, N is even and the ([N/2]+1)th dc output of the modularmultilevel configuration is connected to an electrical ground.

In an embodiment, W is 540 volts.

In an embodiment, V is from 1 kilovolt to 10 kilovolts.

In an embodiment, V is from 1 kilovolt to 3 kilovolts.

In an embodiment, the first set of N ac-dc converter circuits arebidirectional ac-dc converter circuits.

In an embodiment, the first set of N ac-dc converter circuits compriseH-bridges.

In an embodiment, the set of dc-dc converter circuits comprisephase-shifted full bridges.

In an embodiment, the electrical system further comprises:

a second set of N ac-dc converter circuits each having a respectiveindex n=(1, . . . , N), wherein for all n, a dc interface of the nth oneof the second set of ac-dc converter circuits is connected with a seconddc interface of the nth dc-dc converter circuits;

a second rotary electric machine having N independent phases each havinga respective index n=(1, . . . , N), wherein for all n, the nth phase ofthe second electric machine is connected with an ac interface of the nthone of the second set of ac-dc converter circuits.

In an embodiment, the electrical system further comprises:

a second set of N ac-dc converter circuits each having a respectiveindex n=(1, . . . , N), wherein for all n, a dc interface of the nth oneof the second set of ac-dc converter circuits is connected with a seconddc interface of the nth dc-dc converter circuits;

a second electric machine having N polyphase winding sets, each having arespective index n=(1, . . . , N), wherein for all n, the nth windingset of the second electric machine is connected with an ac interface ofthe nth one of the second set of ac-dc converter circuits.

In an embodiment, the second set of N ac-dc converter circuits arebidirectional ac-dc converter circuits.

In an embodiment, the second set of N ac-dc converter circuits compriseH-bridges.

In another aspect, there is provided a gas turbine engine having alow-pressure spool and a high-pressure spool, and further comprising anelectrical system as set out herein, in which the first rotary electricmachine is connected with the low-pressure spool and the second rotaryelectric machine is connected with the high-pressure spool.

In another aspect, there is provided:

a first gas turbine engine having a first spool;

a second gas turbine engine different from the first gas turbine engine,and having a second spool;

and further comprising an electrical system as set out herein, in whichthe first rotary electric machine is connected with the first spool andthe second rotary electric machine is connected with the second spool.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only with referenceto the accompanying drawings, which are purely schematic and not toscale, and in which:

FIG. 1 shows general arrangement of an engine for an aircraft, includingan electric machine on each spool;

FIG. 2 shows an electrical system suitable for the engine of FIG. 1;

FIGS. 3A and 3B show, respectively, a first embodiment of the electricalsystem and a first electric machine therefor;

FIGS. 4A and 4B show, respectively, a second embodiment of theelectrical system and a first electric machine therefor;

FIGS. 5A and 5B show, respectively, a third embodiment of the electricalsystem and a first electric machine therefor;

FIG. 6 shows an embodiment of the electrical system based on that ofFIG. 5A;

FIG. 7 shows another embodiment of the electrical system based on thatof FIG. 5A;

FIG. 8 shows an embodiment of one of the ac-dc converter circuits;

FIG. 9 shows an embodiment of one of the dc-dc converter circuits;

FIG. 10 shows an embodiment of one of the second set of ac-dc convertercircuits.

DETAILED DESCRIPTION FIG. 1

A general arrangement of an engine 101 for an aircraft is shown inFIG. 1. In the present embodiment, the engine 101 is of turbofanconfiguration, and thus comprises a ducted fan 102 that receives intakeair A and generates two pressurised airflows: a bypass flow B whichpasses axially through a bypass duct 103 and a core flow C which entersa core gas turbine.

The core gas turbine comprises, in axial flow series, a low-pressurecompressor 104, a high-pressure compressor 105, a combustor 106, ahigh-pressure turbine 107, and a low-pressure turbine 108.

In operation, the core flow C is compressed by the low-pressurecompressor 104 and is then directed into the high-pressure compressor105 where further compression takes place. The compressed air exhaustedfrom the high-pressure compressor 105 is directed into the combustor 106where it is mixed with fuel and the mixture is combusted. The resultanthot combustion products then expand through, and thereby drive, thehigh-pressure turbine 107 and thereafter the low-pressure turbine 108before being exhausted through a core nozzle 109 to provide a proportionof the overall thrust.

The high-pressure turbine 107 drives the high-pressure compressor 105via an interconnecting shaft. The low-pressure turbine 108 drives thelow-pressure compressor 104 via another interconnecting shaft. Together,the high-pressure compressor 105, high-pressure turbine 107, andassociated interconnecting shaft form a high-pressure spool of theengine 101. Similarly, the low-pressure compressor 104, low-pressureturbine 108, and associated interconnecting shaft form a low-pressurespool of the engine 101. Such nomenclature will be familiar to thoseskilled in the art.

In the present embodiment, the engine 101 is a geared turbofan, wherebythe fan 102 is driven by the low-pressure turbine 108 via a reductiongearbox 110. In the present embodiment, the reduction gearbox 110 is anepicyclic gearbox. In the particular configuration illustrated in FIG.1, the epicyclic gearbox is a planetary-configuration epicyclic gearbox.Thus, the low-pressure turbine 108 is connected with a sun gear of thegearbox 110, which is meshed with a plurality of planet gears located ina rotating carrier, which planet gears are in turn are meshed with astatic ring gear. The rotating carrier drives the fan 102.

It will be appreciated that in alternative embodiments astar-configuration epicyclic gearbox (in which the planet carrier isstatic and the ring gear rotates and provides the output) may be usedinstead. Furthermore, different reduction gearbox configurations couldalso be used, such as step-aside, layshaft, etc.

In the present embodiment, a first rotary electric machine 111 ismechanically coupled with the low-pressure spool. In this specificembodiment, the first electric machine 111 is mounted in the tail cone112 of the engine 101 coaxially with the turbomachinery and is coupledto the low-pressure turbine 108. In an embodiment, the first electricmachine 111 is configured to operate as a motor to drive thelow-pressure spool, facilitating, for example, rotation of the fan 102by electric power alone or electrical augmentation of said rotation. Inanother embodiment, the first electric machine 111 is configured tooperate as a generator to provide electrical power. In the presentembodiment, the first electric machine 111 is configured as amotor-generator allowing both power extraction from and power injectionto the low-pressure spool. The mode of operation may vary depending onflight phase to improve operability, fuel consumption, etc.

In alternative embodiments, the first electric machine 111 may belocated axially in line with low-pressure compressor 104, which mayadopt a bladed disc or bladed drum configuration to provide space forthe first electric machine 111.

In the present embodiment, a second rotary electric machine 113 ismechanically coupled with the high-pressure spool. In this specificembodiment, the second electric machine 113 is coupled to thehigh-pressure spool via a high-pressure spool driven, core-mountedaccessory gearbox 114 of conventional drive configuration, for examplevia a tower-shaft. In an embodiment, the second electric machine 113 isconfigured to operate as a motor to drive the high-pressure spool,facilitating, for example, starting of the engine 101. In anotherembodiment, the second electric machine 113 is configured to operate asa generator to provide electrical power. In the present embodiment, thesecond electric machine 113 is configured as a motor-generator allowingboth power extraction from and power injection to the high-pressurespool. The mode of operation may vary depending on flight phase toimprove operability, fuel consumption, etc.

In alternative embodiments, the second electric machine 113 may bemounted coaxially with the turbomachinery in the engine 101. Forexample, the second electric machine 113 may be mounted axially in linewith the duct between the low- and high-pressure compressors 104 and105.

It will of course be appreciated by those skilled in the art that anyother suitable location for the first and second electric machines maybe adopted.

In the present embodiment, the first and second electric machines areconnected with power electronics. Extraction of power from, orapplication of power to the electric machines is performed by a powerelectronics module (PEM) 115. In the present embodiment, the PEM 115 ismounted on the fan case 116 of the engine 101, but it will beappreciated that it may be mounted elsewhere such as on the core of thegas turbine, or in the vehicle to which the engine 101 is attached, forexample. Further, different parts of the PEM 116 may be distributedbetween different locations. For example, some components may be mountedon the engine 101 and some may be mounted in the in the vehicle to whichthe engine 101 is attached.

Control of the PEM 115 and thus of the first and second electricmachines 111 and 113 is in the present example performed by anelectronic engine controller (EEC) 117. In the present embodiment theEEC 117 is a full-authority digital engine controller (FADEC), theconfiguration of which will be known and understood by those skilled inthe art. It therefore controls all aspects of the engine 101, i.e. bothof the core gas turbine and the first and second electric machines 111and 113. In this way, the EEC 117 may holistically respond to boththrust demand and electrical power demand.

As set out previously, in the present embodiment the first electricmachine 111 and the second electric machine 113 may both be configuredas motor-generators. The mode of operation of each of the first electricmachine 111 and the second electric machine 113 may thus be adjusted inconcert to transfer power to and from the high- and low-pressure spools.In this way, the turbomachinery may be designed to exploit the attendantadvantages conferred by transfer of power between the high-pressurespool and the low-pressure spool. For example, transfer of power fromthe low-pressure spool to the high-pressure spool during the approachphase reduces the effective thrust of the engine 101 whilst maintainingsufficient high-pressure spool rotational speed to safely initiate ago-around manoeuvre. Further, in engine 101, transfer of power from thehigh-pressure spool to the low-pressure spool during a decelerationmanoeuvre reduces the risk of weak extinction, therefore enabling a moreoptimal combustor design.

The configuration and operation of the electrical system will bedescribed with reference to FIG. 2, and the configuration of theelectric machines will be described further with reference to FIGS. 3Aand 3B.

Various embodiments of the engine 101 may include one or more of thefollowing features.

It will be appreciated that instead of being a turbofan having a ductedfan arrangement, the engine 101 may instead be a turboprop comprising apropeller for producing thrust.

The low- and high-pressure compressors 104 and 105 may comprise anynumber of stages, for example multiple stages. Each stage may comprise arow of rotor blades and a row of stator vanes, which may be variablestator vanes (in that their angle of incidence may be variable). Inaddition to, or in place of, axial stages, the low- or high-pressurecompressors 104 and 105 may comprise centrifugal compression stages.

The low- and high-pressure turbines 107 and 108 may also comprise anynumber of stages.

The fan 102 may have any desired number of fan blades, for example 16,18, 20, or 22 fan blades.

Each fan blade may be defined as having a radial span extending from aroot (or hub) at a radially inner gas-washed location, or 0 percent spanposition, to a tip at a 100 percent span position. The ratio of theradius of the fan blade at the hub to the radius of the fan blade at thetip—the hub-tip ratio—may be less than (or on the order of) any of: 0.4,0.39, 0.38 0.37, 0.36, 0.35, 0.34, 0.33, 0.32, 0.31, 0.3, 0.29, 0.28,0.27, 0.26, or 0.25. The hub-tip ratio may be in an inclusive rangebounded by any two of the aforesaid values (i.e. the values may formupper or lower bounds). The hub-tip ratio may both be measured at theleading edge (or axially forwardmost) part of the blade. The hub-tipratio refers, of course, to the gas-washed portion of the fan blade,i.e. the portion radially outside any platform.

The radius of the fan 102 may be measured between the engine centrelineand the tip of a fan blade at its leading edge. The fan diameter may begreater than (or on the order of) any of: 2.5 metres, 2.6 metres, 2.7metres, 2.8 metres, 2.9 metres, 3 metres, 3.1 metres, 3.2 metres, 3.3metres, 3.4 metres, 3.5 metres, 3.6 metres, 3.7 metres, 3.8 metres or3.9 metres. The fan diameter may be in an inclusive range bounded by anytwo of the aforesaid values (i.e. the values may form upper or lowerbounds).

The rotational speed of the fan 102 may vary in use. Generally, therotational speed is lower for fans with a higher diameter. Purely by wayof non-limitative example, the rotational speed of the fan at cruiseconditions may be less than 2500 rpm, for example 2300 rpm. Purely byway of further non-limitative example, the rotational speed of the fan102 at cruise conditions for an engine having a fan diameter in therange of from 2.5 metres to 3 metres (for example 2.5 metres to 2.8metres) may be in the range of from 1700 rpm to 2500 rpm, for example inthe range of from 1800 rpm to 2300 rpm, or, for example in the range offrom 1900 rpm to 2100 rpm. Purely by way of further non-limitativeexample, the rotational speed of the fan at cruise conditions for anengine having a fan diameter in the range of from 3.2 metres to 3.8metres may be in the range of from 1200 rpm to 2000 rpm, for example inthe range of from 1300 rpm to 1800 rpm, for example in the range of from1400 rpm to 1600 rpm.

In use of the engine 101, the fan 102 (with its associated fan blades)rotates about a rotational axis. This rotation results in the tip of thefan blade moving with a velocity U_(tip). The work done by the fanblades on the flow results in an enthalpy rise dH of the flow. A fan tiploading may be defined as dH/U_(tip) ², where dH is the enthalpy rise(for example the one-dimensional average enthalpy rise) across the fanand U_(tip) is the (translational) velocity of the fan tip, for exampleat the leading edge of the tip (which may be defined as fan tip radiusat leading edge multiplied by angular speed). The fan tip loading atcruise conditions may be greater than (or on the order of) any of: 0.3,0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39 or 0.4. The fan tiploading may be in an inclusive range bounded by any two of the values inthe previous sentence (i.e. the values may form upper or lower bounds).

The engine 101 may have any desired bypass ratio, where the bypass ratiois defined as the ratio of the mass flow rate of the flow B through thebypass duct to the mass flow rate of the flow C through the core atcruise conditions. Depending upon the selected configuration, the bypassratio may be greater than (or on the order of) any of the following: 10,10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14, 14.5, 15, 15.5, 16, 16.5, or 17.The bypass ratio may be in an inclusive range bounded by any two of theaforesaid values (i.e. the values may form upper or lower bounds). Thebypass duct may be substantially annular. The bypass duct may beradially outside the core engine. The radially outer surface of thebypass duct may be defined by a nacelle and/or a fan case.

The overall pressure ratio of the engine 101 may be defined as the ratioof the stagnation pressure upstream of the fan 102 to the stagnationpressure at the exit of the high-pressure compressor 105 (before entryinto the combustor). By way of non-limitative example, the overallpressure ratio of the engine 101 at cruise may be greater than (or onthe order of) any of the following: 35, 40, 45, 50, 55, 60, 65, 70, 75.The overall pressure ratio may be in an inclusive range bounded by anytwo of the aforesaid values (i.e. the values may form upper or lowerbounds).

Specific thrust of the engine 101 may be defined as the net thrust ofthe engine divided by the total mass flow through the engine 101. Atcruise conditions, the specific thrust of the engine 101 may be lessthan (or on the order of) any of the following: 110 Nkg⁻¹s, 105 Nkg⁻¹s,100 Nkg⁻¹s, 95 Nkg⁻¹s, 90 Nkg⁻¹s, 85 Nkg⁻¹s, or 80 Nkg⁻¹s. The specificthrust may be in an inclusive range bounded by any two of the values inthe previous sentence (i.e. the values may form upper or lower bounds).Such engines may be particularly efficient in comparison withconventional gas turbine engines.

The engine 101 may have any desired maximum thrust. For example, theengine 101 may be capable of producing a maximum thrust of at least (oron the order of) any of the following: 160 kilonewtons, 170 kilonewtons,180 kilonewtons, 190 kilonewtons, 200 kilonewtons, 250 kilonewtons, 300kilonewtons, 350 kilonewtons, 400 kilonewtons, 450 kilonewtons, 500kilonewtons, or 550 kilonewtons. The maximum thrust may be in aninclusive range bounded by any two of the aforesaid values (i.e. thevalues may form upper or lower bounds). The thrust referred to above maybe the maximum net thrust at standard atmospheric conditions at sealevel plus 15 degrees Celsius (ambient pressure 101.3 kilopascals,temperature 30 degrees Celsius), with the engine 101 being static.

In use, the temperature of the flow at the entry to the high-pressureturbine 107 may be particularly high. This temperature, which may bereferred to as turbine entry temperature or TET, may be measured at theexit to the combustor 106, for example immediately upstream of the firstturbine vane, which itself may be referred to as a nozzle guide vane. Atcruise, the TET may be at least (or on the order of) any of thefollowing: 1400 kelvin, 1450 kelvin, 1500 kelvin, 1550 kelvin, 1600kelvin or 1650 kelvin. The TET at cruise may be in an inclusive rangebounded by any two of the aforesaid values (i.e. the values may formupper or lower bounds). The maximum TET in use of the engine 101 may be,for example, at least (or on the order of) any of the following: 1700kelvin, 1750 kelvin, 1800 kelvin, 1850 kelvin, 1900 kelvin, 1950 kelvinor 2000 kelvin. The maximum TET may be in an inclusive range bounded byany two of the aforesaid values (i.e. the values may form upper or lowerbounds). The maximum TET may occur, for example, at a high thrustcondition, for example at a maximum take-off (MTO) condition.

A fan blade and/or aerofoil portion of a fan blade described and/orclaimed herein may be manufactured from any suitable material orcombination of materials. For example, at least a part of the fan bladeand/or aerofoil may be manufactured at least in part from a composite,for example a metal matrix composite and/or an organic matrix composite,such as carbon fibre. By way of further example at least a part of thefan blade and/or aerofoil may be manufactured at least in part from ametal, such as a titanium-based metal or an aluminium based material(such as an aluminium-lithium alloy) or a steel-based material. The fanblade may comprise at least two regions manufactured using differentmaterials. For example, the fan blade may have a protective leadingedge, which may be manufactured using a material that is better able toresist impact (for example from birds, ice or other material) than therest of the blade. Such a leading edge may, for example, be manufacturedusing titanium or a titanium-based alloy. Thus, purely by way ofexample, the fan blade may have a carbon-fibre or aluminium-based bodywith a titanium leading edge.

The fan 102 may comprise a central hub portion, from which the fanblades may extend, for example in a radial direction. The fan blades maybe attached to the central portion in any desired manner. For example,each fan blade may comprise a fixture which may engage a correspondingslot in the hub. Purely by way of example, such a fixture may be in theform of a dovetail that may slot into and/or engage a corresponding slotin the hub/disc in order to fix the fan blade to the hub. By way offurther example, the fan blades maybe formed integrally with a centralhub portion. Such an arrangement may be a bladed disc or a bladed ring.Any suitable method may be used to manufacture such a bladed disc orbladed ring. For example, at least a part of the fan blades may bemachined from a billet and/or at least part of the fan blades may beattached to the hub/disc by welding, such as linear friction welding.

The engine 101 may be provided with a variable area nozzle (VAN). Such avariable area nozzle may allow the exit area of the bypass duct to bevaried in use. The general principles of the present disclosure mayapply to engines with or without a VAN.

As used herein, cruise conditions have the conventional meaning andwould be readily understood by those skilled in the art.

Such cruise conditions may be conventionally defined as the conditionsat mid-cruise, for example the conditions experienced by the aircraftand/or engine at the midpoint (in terms of time and/or distance) betweentop of climb and start of descent. Cruise conditions thus define anoperating point of the gas turbine engine which provides a thrust thatwould ensure steady state operation (i.e. maintaining a constantaltitude and constant Mach number) at mid-cruise of an aircraft to whichit is to designed to be attached, taking into account the number ofengines provided to that aircraft. For example, where an engine isdesigned to be attached to an aircraft that has two engines of the sametype, at cruise conditions the engine provides half of the total thrustthat would be required for steady state operation of that aircraft atmid-cruise.

In other words, for a given gas turbine engine for an aircraft, cruiseconditions are defined as the operating point of the engine thatprovides a specified thrust (required to provide—in combination with anyother engines on the aircraft—steady state operation of the aircraft towhich it is designed to be attached at a given mid-cruise Mach number)at the mid-cruise atmospheric conditions (defined by the InternationalStandard Atmosphere according to ISO 2533 at the mid-cruise altitude).For any given gas turbine engine for an aircraft, the mid-cruise thrust,atmospheric conditions and Mach number are known, and thus the operatingpoint of the engine at cruise conditions is clearly defined.

The cruise conditions may correspond to ISA standard atmosphericconditions at an altitude that is in the range of from 10000 to 15000metres, such as from 10000 to 12000 metres, or from 10400 to 11600metres (around 38000 feet), or from 10500 to 11500 metres, or from 10600to 11400 metres, or from 10700 metres (around 35000 feet) to 11300metres, or from 10800 to 11200 metres, or from 10900 to 11100 metres, or11000 metres. The cruise conditions may correspond to standardatmospheric conditions at any given altitude in these ranges.

The forward speed at the cruise condition may be any point in the rangeof from Mach 0.7 to 0.9, for example one of Mach 0.75 to 0.85, Mach 0.76to 0.84, Mach 0.77 to 0.83, Mach 0.78 to 0.82, Mach 0.79 to 0.81, Mach0.8, Mach 0.85, or in the range of from Mach 0.8 to 0.85. Any singlespeed within these ranges may be the cruise condition. For someaircraft, the cruise conditions may be outside these ranges, for examplebelow Mach 0.7 or above Mach 0.9.

Thus, for example, the cruise conditions may correspond specifically toa pressure of 23 kilopascals, a temperature of minus 55 degrees Celsius,and a forward Mach number of 0.8.

It will of course be appreciated, however, that the principles of theinvention claimed herein may still be applied to engines having suitabledesign features falling outside of the aforesaid parameter ranges.

FIG. 2

An electrical system 201 for connecting the first electric machine 111to a first dc network 202 and a second dc network 203 operating atdifferent voltages is shown in FIG. 2. The electrical system 201 isshown in the form of a single line diagram, the conventions of whichwill be familiar to those skilled in the art. Thus for alternatingcurrent (ac) a single line replaces a plurality of polyphase lines, andfor direct current (dc) a single line replaces the +V and −V lines.

In the present embodiment, the first electric machine 111 is connectedwith the first dc network 202 via a first set of ac-dc convertercircuits 204. In the illustrated embodiment, the first set of ac-dcconverter circuits 204 form part of the PEM 115. As set out previously,in the present embodiment the first electric machine 111 is configuredas a motor-generator and thus the first set of ac-dc converter circuits204 are bidirectional ac-dc converter circuits. In a specificembodiment, the ac-dc converter circuits 204 are H-bridge convertercircuits, the topology of which will be described with reference to FIG.7. The ac-dc converter circuits may however be any suitable alternativetopology such as neutral-point clamped, etc. Further, as describedpreviously in alternative embodiments the first electric machine 111 maybe configured solely as a generator or a solely as a motor, in whichcase the first set of ac-dc converter circuits 204 may be configured asunidirectional ac-dc converter circuits.

The first dc network operates at a first voltage V. The second dcnetwork 203 operates at a second voltage W. The first voltage V isgreater than the second voltage W, i.e. V>W. The voltages referred toherein may be a nominal voltage for each dc network, which definition isapplicable when the voltages are substantially fixed. In alternativeembodiments, the voltages referred to herein may be the actualoperational voltages, which definition is appropriate particularly inthe case of variable dc voltage networks.

A set of dc-dc converter circuits 205 is therefore provided to convertdc power between the first voltage V at a first dc interface and thesecond voltage W at a second dc interface so as to allow power to betransferred from the first dc networks 202 to the second dc network 203.In the illustrated embodiment, the set of dc-dc converter circuits 204also form part of the PEM 115. In a specific embodiment, the set ofdc-dc converter circuits 204 comprises phase-shifted full bridgecircuits, the topology of which will be described with reference to FIG.8. The dc-dc converter circuits may however be any suitable alternativetopology which provides isolation between input and output toaccommodate different absolute voltage levels, such as a hard-switchpulse width modulated type, a resonant type, or a soft-switching type.For example, the dc-dc converter circuits may be isolated forwardconverters, or isolated push-pull converters, etc.

In the present embodiment, the electrical system 201 further encompassesthe second electric machine 113. In this configuration, the secondelectric machine 113 is connected to the second dc network 203 via asecond set of ac-dc converters 206.

The configuration of the ac-dc converters and dc-dc converters will bedescribed further with reference to FIGS. 4 to 6.

In the present embodiment, a controller 207 is provided in the EEC 117.The controller 207 is configured to control the operation of theconverter circuits within the PEM 115 so as to control the operation ofthe electric machines 111 and 113 and the transfer of power between thedc networks 202 and 203. In the present example, the controller 207 is afunctional module implemented in software running on the EEC 117. Itwill be appreciated that in alternative embodiments the controller 207may be implemented in hardware in the EEC 117. It will also beappreciated that the controller 207 may be a separate module in additionto the EEC 117.

Whilst the embodiment described thus far is in the context of aninstallation in a single gas turbine engine, it is envisaged that theelectric machines could be connected to spools in different gasturbines. Further, the electrical system could be used in otherapplications such as connection of steam turbines and reciprocatingengines, etc. or in any other suitable application.

FIGS. 3A & 3B

As set out previously, the first electric machine 111 is a rotaryelectric machine having N≥2 independent phases. As will be understood bythose skilled in the art, phase independence facilitates electricalisolation between the phases by driving each phase by its own dedicatedconverter circuit. This principle has been successfully demonstrated inthe form of a drive system for an aircraft fuel pump as described inU.S. Pat. No. 8,823,332, which is assigned to the present applicant.

A first embodiment of the electrical system 201 is shown in FIG. 3A, inwhich N=2. For the purposes of clarity the second electric machine 113and the controller 207 are omitted at this stage. An embodiment of thefirst electric machine 111 for use with the electrical system 201 ofFIG. 3A, is shown in FIG. 3B.

With N being set equal to 2 in this example, the first electric machine111 has two independent phases having an associated index having anassociated index n=(1, 2), identified by the two separate coil symbolsthereon in FIG. 3A. In order to provide the independent phase drivefirst electric machine 111, the first set of ac-dc converter circuits204 comprises N ac-dc converter circuits, each of which has a respectiveindex n=(1, . . . , N). Thus, in this case, two ac-dc converter circuitsare provided: a first ac-dc converter circuit 301 which has an indexn=1, and a second ac-dc converter circuit 302 which has an index n=2.For ease of reference in the Figure, the indices for each ac-dcconverter circuit and other indexed elements are shown as enclosedalphanumeric characters.

For all n, an ac interface of each ac-dc converter circuit is connectedwith the nth phase of the first electric machine 111: thus an acinterface the first ac-dc converter circuit 301—which has index n=1—isconnected with phase 1 of the first electric machine 111, and an acinterface the first ac-dc converter circuit 301—which has index n=2—isconnected with phase 2.

The ac interfaces of first set of ac-dc converter circuits 204 areconnected in such a way as to form a modular multilevel (MML)configuration having, in general terms, P=N+1 dc outputs each having arespective index p=(1, . . . , P). Thus, in this example, P=3. The firstdc output, with index p=1, is provided by the lower-level voltage outputof the first ac-dc converter circuit 301. The third dc output, withindex p=3, is provided by the higher-level voltage output of the secondac-dc converter circuit 302.

The second dc output, with index p=2, is formed by connecting thehigh-level voltage output of the first ac-dc converter circuit 301 withthe low-level dc voltage output of the second ac-dc converter circuit302. This means that the voltage at these outputs of the ac-dc convertercircuits are forced to be equal. Consequently, the overall dc outputvoltage of the MML configuration is the sum of the voltages across therespective lower- and higher-level voltage outputs of each convertercircuit therein. In the present embodiment, the potential differenceacross the outputs of each converter circuit is the same.

In the present example, the overall dc output voltage of the first setof ac-dc converter circuits 204 is a voltage V. Thus, the potentialdifference between the pth dc output and the (p+q)th dc output is qV/N,where, as before, P=N+1, p=(1, . . . , P), and here q=(0, . . . , P−p).

Thus, in the example of FIG. 3A, the potential difference between thefirst dc output of the MML configuration (having an index of 1) and thesecond dc output (having an index of 2) is V/2, as in that case p=1,q=1, and N=2. The potential difference between the first output of themodular multilevel configuration and the third output (having an indexof 3) is V, as in that case p=1, q=2, and N=2.

A supply for the first dc network 202 may therefore be taken across afirst node 303 at the first dc output from the MML configuration, and asecond node 304 at the third dc output from the MML configuration.

In the present embodiment, it will be seen that N is even and thus thesecond output from the modular multilevel configuration can represent aneutral point. Thus, in the present embodiment, a third node 305 at thesecond output from the MML configuration is connected to an electricalground. This could be a true ground or a floating neutral in an ITgrounding scheme, or any other suitable ground type. In general terms,therefore, those skilled in the art will see that for all systems whereN is even, it is possible to connect the ([N/2]+1)th dc output of theMML configuration to such a ground point.

In a practical example, the voltage V may be from 1 kilovolt to 10kilovolts. More specifically, V may be from 1 kilovolt to 3 kilovolts.

In order to convert the voltage V of the first dc network 202 to thevoltage W of the second dc network 203, the set of dc-dc convertercircuits 205 comprises N dc-dc converter circuits each having arespective index n=(1, . . . , N). Each of these dc-dc convertercircuits is configured to convert dc power between a voltage V/N at afirst dc interface and a voltage W at a second dc interface. For all n,a first dc interface of the nth dc-dc converter circuit is connectedwith the dc outputs of the MML configuration whose index p is equal to nand (n+1). In a practical example, voltage W is 540 volts.

In the example of FIG. 3A, the set of dc-dc converter circuits 205comprises a first dc-dc converter circuit 306 which has an index n=1,and a second dc-dc converter circuit 307 which has an index n=2.

A first dc interface of the first dc-dc converter circuit 306 isconnected with the first and second dc outputs of the MML configuration,i.e. those outputs with indices p equal to n and n+1. Similarly, a firstdc interface of the second dc-dc converter circuit 307 is connected withthe second and third dc outputs of the MML configuration. The firstdc-dc converter circuit 306 and the second dc-dc converter circuit 307both output voltage W at their second dc interfaces for output to thesecond dc network 203.

In the present example, each dc-dc converter circuit must only convertbetween a voltage V/2 and a voltage W, rather than a voltage V and avoltage W. It will be appreciated by those skilled in the art that asthe difference between voltage V and voltage W increases, the approachof the present invention reduces the difference in voltage across thedc-dc converter circuits, thereby reducing losses, mass and volume.

A first embodiment of the first electric machine 111 is shown in FIG.3B, which suitable for use in the system of FIG. 3A.

In this specific embodiment, the first electric machine 111 comprises astator 311 having eight slots 312 defined by teeth 313. The stator 311has a single-layer concentrated winding arrangement having four coils314, 315, 316, and 317. As indicated in the Figure, coils 314 and 316are connected as part of phase 1, whilst coils 315 and 317 are connectedas part of phase 2. In the present embodiment the coils forming part ofthe same phase are connected in parallel, however they may also beconnected in series.

The coils 314 to 317 are wound on alternate teeth to provide physicalisolation between the phases, improving fault tolerance. The teeth notcarrying a coil are often referred to as spacer teeth. In an embodiment,the wound teeth and the spacer teeth are equal in width. In alternativeembodiments, the wound teeth may be wider than the spacer teeth. As eachphase comprises two coils separated mechanically by 180 degrees,mechanical balance is improved. Alternative arrangements with half thenumber of slots and coils may be used though where balance is not asimportant as other considerations, for example size or weight. On theother hand, more slots and coils could be provided for further improvedperformance.

In alternative embodiments, a winding arrangement could be adopted inwhich each tooth carries a coil, i.e. the stator has no spacer teeth.This may improve power density in applications where less faulttolerance is required. Furthermore, in alternative embodiments, adistributed winding arrangement may be adopted. Spacer teeth may be usedin such an arrangement to provide a degree of fault tolerance byisolation of coils. Should this aspect of fault tolerance not berequired by the application, then spacer teeth may be omitted.

In the present embodiment, the rotor 318 is a permanent magnet rotor,however the principles disclosed herein may be applied to other machinetypes such as synchronous reluctance machines.

FIGS. 4A & 4B

A second embodiment of the electrical system 201 in which N=3 is shownin FIG. 4A.

As with the embodiment described with reference to FIG. 3A, the firstset of ac-dc converter circuits 204 comprises ac-dc converter circuitsconnected in an MML configuration. In this example, N=3 and thus threeac-dc converter circuits are provided, each having an associated indexn: a first ac-dc converter circuit 401 having an index 1, a second ac-dcconverter circuit 402 having an index 2, and a third ac-dc convertercircuit 403 having an index 3. The ac-side of each ac-dc convertercircuit drives a respective phase having an associated index n=(1, 2, 3)in the electric machine 111.

The MML configuration has, in this embodiment, P=4 outputs each havingan associated index p. As described previously with reference to FIG.3A, in general terms the potential difference between the pth output andthe (p+q)th output of the MML configuration is qV/N, where q=(0, . . . ,P−p). Thus in the specific example of FIG. 4A, the potential differencebetween the pth output and the (p+q)th output is qV/3, q=(0, . . . ,P−p). The potential difference between the first and second dc output istherefore V/3; between the first and third dc output is 2V/3; andbetween the first and fourth dc output it is V.

It will be seen that in this example, N is odd and therefore there is noneutral point formed by the MML configuration, and thus no groundconnection is made in this embodiment. In a different embodiment, a highimpedance ground may be achieved by using a passive network such as anelectrical filter or a resistive divider to ground, for example.

In the present example, the first dc network 202 is connected to a firstnode 404 at the first dc output of the MML configuration (index p=1),and a second node 405 at the fourth dc output (index p=4) of the MMLconfiguration.

Conversion of voltage for the second dc network 203 is again performedby the set of dc-dc converter circuits 205, which in this embodimentcomprises a first dc-dc converter circuit 406 which has an index n=1, asecond dc-dc converter circuit 407 which has an index n=2, and a thirddc-dc converter circuit 408 which has an index n=3. As with theembodiment of the FIG. 3A, a first dc interface of the nth dc-dcconverter circuit is connected with the nth and (n+1)th dc outputs ofthe modular multilevel configuration. Each dc-dc converter circuit 406to 408 outputs voltage W at their second dc interfaces for output to thesecond dc network 203.

In this example, each dc-dc converter circuit must only convert betweena voltage V/3 and a voltage W.

A second embodiment of the first electric machine 111 is shown in FIG.4B, which suitable for use in the system of FIG. 4A.

In this specific embodiment, the first electric machine 111 comprises astator 411 having twelve slots 412 defined by teeth 413. The stator 411has a single-layer concentrated winding arrangement having six coils414, 415, 416, 417, 418, and 419. As indicated in the Figure, coils 414and 417 are connected as part of phase 1, coils 415 and 418 areconnected as part of phase 2, and coils 416 and 419 are connected aspart of phase 3. In the present embodiment the coils forming part of thesame phase are connected in parallel, however they may also be connectedin series.

The coils 414 to 419 are wound on alternate teeth to provide physicalisolation between the phases, improving fault tolerance. As each phasecomprises two coils separated by 180 degrees, mechanical balance isimproved. Alternative arrangements with half the number of slots andcoils may be used though where balance is not as important as otherconsiderations, for example size or weight. On the other hand, moreslots and coils per phase could be provided for further improvedperformance.

Again, in alternative embodiments, a winding arrangement could beadopted in which each tooth carries a coil, i.e. the stator has nospacer teeth. This may improve power density in applications where lessfault tolerance is required.

Again, in this embodiment, the rotor 420 is a permanent magnet rotor,however the principles disclosed herein may be applied to other machinetypes such as synchronous reluctance machines.

FIGS. 5A & 5B

A third embodiment of the electrical system 201 in which N=4 is shown inFIG. 5A.

As with the embodiments described with reference to FIGS. 3A and 4A, thefirst set of ac-dc converter circuits 204 comprises ac-dc convertercircuits connected in an MML configuration. In this example, N=4 andthus four ac-dc converter circuits are provided, each having anassociated index n: a first ac-dc converter circuit 501 having an index1, a second ac-dc converter circuit 502 having an index 2, a third ac-dcconverter circuit 503 having an index 3, and a fourth ac-dc convertercircuit 504 having an index 4. The ac-side of each ac-dc convertercircuit drives a respective phase having an associated index n=(1, . . ., 4) in the electric machine 111.

The MML configuration has, in this embodiment, P=5 outputs each havingan associated index p. In this specific example, the potentialdifference between the pth output and the (p+q)th output is qV/4, q=(0,. . . , P−p). The potential difference between the first and second dcoutput is therefore V/4; between the first and third dc output is V/2;between the first and fourth dc output it is 3V/4; and between the firstand fifth dc output it is V.

It will be seen that in this example, N is even and therefore a neutralpoint is formed at the third output of the MML configuration, allowingan optional ground connection to be made at a node 505 connectedtherewith.

In the present example, the first dc network 202 is connected to a firstnode 506 at the first dc output of the MML configuration (index p=1),and a second node 507 at the fifth dc output (index p=5) of the MMLconfiguration.

Conversion of voltage for the second dc network 203 is again performedby the set of dc-dc converter circuits 205, which in this embodimentcomprises a first dc-dc converter circuit 508 which has an index n=1, asecond dc-dc converter circuit 509 which has an index n=2, a third dc-dcconverter circuit 510 which has an index n=3, and a fourth dc-dcconverter circuit 511 which has an index n=4. As with the embodiment ofFIGS. 3A and 4A, a first dc interface of the nth dc-dc converter circuitis connected with the nth and (n+1)th dc outputs of the modularmultilevel configuration. Each dc-dc converter circuit 508 to 511outputs voltage W at their second dc interfaces for output to the seconddc network 203.

In this example, each dc-dc converter circuit must only convert betweena voltage V/4 and a voltage W. Thus, in example, V may be 3 kilovolts,and W may be 540 volts. The dc-dc converter circuits therefore only needto be configured to convert between 750 volts and 540 volts.

The first embodiment of the first electric machine 111 is shown in FIG.5B, albeit this time it is adapted for use in the third embodiment ofthe electrical system 201 shown in FIG. 5A.

In this embodiment, each coil is connected with a respective phase, withno parallel or series connections between coils. Thus, coil 314 formspart of phase 1 as indicated thereon, coil 315 forms part of phase 2,coil 316 forms part of phase 3, and coil 317 forms part of phase 4.

In alternative embodiments, the first electric machine 111 could beconfigured with double the number of slots and coils, with coilsseparated by 180 degrees forming part of one of the four phases. Thiswould ensure mechanical balance in the event of failure of one of theac-dc converter circuits.

FIG. 6

A fourth embodiment of the electrical system 201 is shown FIG. 6, whichbuilds upon the embodiment of FIG. 5A. Like features are thereforeidentified with like reference numerals.

In this embodiment, the second electric machine 113 also has Nindependent phases each having a respective index n=(1, . . . , N). Forall n, the nth phase of the second electric machine 113 is connectedwith an ac interface of the nth one of the second set of ac-dc convertercircuits 206. In this example therefore, the second electric machine 113comprises windings connected to form N=4 independent phases each havinga respective index n of from 1 to 4. In an embodiment, the secondelectric machine 113 may be of the same configuration as the firstelectric machine 111. In other embodiments, it may differ inconfiguration, for example number of coils per phase, number of teeth,alternate wound coils or not, etc.

The second set of ac-dc converter circuits 206 comprises N ac-dcconverter circuits each having a respective index n=(1, . . . , N). Forall n, a dc interface of the nth one of the second set of ac-dcconverter circuits 206 is connected with a second dc interface of thenth one of the set of dc-dc converters 205. In an embodiment, the secondset of ac-dc converter circuits 206 are configured in the same way asthe first set of ac-dc converter circuits 204. In an embodiment, theymay be configured different from the first set of ac-dc convertercircuits 204. In an embodiment they are unidirectional inverters. Inanother embodiment, they are unidirectional rectifiers. In anotherembodiment, they are bidirectional converters. They may be H-bridge,NPC, or any other suitable topology.

In this example, with N=4, the second set of ac-dc converter circuits206 comprises a first ac-dc converter circuit 601 having an index n=1, asecond ac-dc converter circuit 602 having an index n=2, a third ac-dcconverter circuit 603 having an index n=3, and a fourth ac-dc convertercircuit 604 having an index n=4. The dc interface of the nth one of thesecond set of ac-dc converter circuits is connected with a second dcinterface of the nth dc-dc converter circuit—thus the first ac-dcconverter circuit 601 is connected with the second dc interface of thefirst dc-dc converter 508, and so on.

In this way, power transfer may be achieved between the electricmachines and dc networks, despite the difference in voltage V and W.

FIG. 7

A fifth embodiment of the electrical system 201 is shown FIG. 7, whichalso builds upon the embodiment of FIG. 5A. Like features are thereforeidentified with like reference numerals.

In this embodiment, the second electric machine 113 comprises Npolyphase winding sets, each having a respective index n=(1, . . . , N).As used herein, the term polyphase winding set refers to a number M≥2 ofwindings of any suitable configuration which are excited by and/orproduce a balanced system of M polyphase voltages, i.e. identicalvoltage waveforms offset by 2π/M radians. For example, the winding setmay be a three phase, star connected winding set excited by a polyphasesupply. One or more windings may form part of each phase. Deltaconnection is also possible. The winding set could comprise any othernumber of phases.

The presence of N polyphase winding sets provides redundancy, whereby ifthe drive circuit for a particular polyphase winding set fails,continued operation may be provided by the other winding set(s). In anembodiment, the second electric machine 113 is an N-tuple wound electricmachine, in which each of the N polyphase winding sets are wound on thesame stator around a common rotor. In another embodiment, the secondelectric machine 113 is an N-tuple stacked machine, in which a commonrotor is shared between N coaxially connected stators each carrying oneof the N polyphase winding sets.

In this example, the second set of ac-dc converter circuits 206comprises N ac-dc converter circuits each having a respective indexn=(1, . . . , N). For all n, the nth winding set of the second electricmachine 113 is connected with an ac interface of the nth one of thesecond set of ac-dc converter circuits 206. In this configuration, eachof second set of ac-dc converter circuits 206 is configured to provide apolyphase supply to the corresponding winding set in the second electricmachine 113. The dc interface of the nth one of the second set of ac-dcconverter circuits 206 is connected with a second dc interface of thenth one of the set of dc-dc converter circuits 205.

Thus, in the example of FIG. 7, the second electric machine 113comprises N=4 winding sets, identified with indices 1 to 4. In thisexample the polyphase winding sets comprise three phases joined in astar connection. As described previously, a delta connection is alsopossible, as is a different number of phases. In the present example,the second electric machine 113 is configured with a quadruple (4-tuple)wound stator and a single permanent-magnet rotor. It will be appreciatedthat other configurations are possible as set out previously.

In this example, the second set of ac-dc converter circuits 206comprises a first ac-dc converter circuit 701 having an index n=1, asecond ac-dc converter circuit 702 having an index n=2, a third ac-dcconverter circuit 703 having an index n=3, and a fourth ac-dc convertercircuit 704 having an index n=4. The dc interface of the nth one of thesecond set of ac-dc converter circuits is connected with a second dcinterface of the nth dc-dc converter circuit—thus the first ac-dcconverter circuit 701 is connected with the second dc interface of thefirst dc-dc converter 508, and so on.

FIG. 8

An embodiment of one of the ac-dc converter circuits is shown in FIG. 8.This circuit would be suitable for use as one of the first set of ac-dcconverter circuits 204 in any of the embodiments as heretoforedescribed, and for use as one of the second set of ac-dc convertercircuits 206 in the embodiment of FIG. 6.

Those skilled in the art will recognise the ac-dc converter circuit 801as an H-bridge circuit, having ac-side terminals 802 and 803 forming anac interface for connection with phase windings of an electric machine,and dc-side terminals 804 and 805 which form a dc interface. In thisexample, the dc-side terminal 804 provides the higher-level voltageoutput, whilst the other dc-side terminal 805 provides the lower-levelvoltage output. In this example, control of the switches is performed bya local embedded controller within the ac-dc converter circuit 801 (notshown), suitable types of which will be familiar to those skilled in theart. Local embedded controllers of this type typically accept speed,position, voltage, and/or torque references from the controller 207, andoperate gate drives to effect pulse-width modulation-based control ofthe circuit, a technique which will be familiar to those skilled in theart.

FIG. 9

An embodiment of one of the dc-dc converter circuits is shown in FIG. 9.This circuit would be suitable for use as one of the set of dc-dcconverter circuits 205 in any of the embodiments as heretoforedescribed.

Those skilled in the art will recognise the dc-dc converter circuit 901as a phase-shifted full bridge circuit, having a first pair of terminals902 and 903 forming a first dc interface and a second pair of terminals904 and 905 which form a second dc interface. In this example, fourMOSFET switches 906, 907, 908, and 909 form a full bridge on the primaryside of a transformer 910. Two MOSFET switches 911 and 912 providepush-pull switching on the secondary side of the transformer 910. Thecircuit may operate in buck or boost mode to effect conversion fromvoltage V/N to voltage W and vice versa. As will be appreciated by thoseskilled in the art, fast-acting diodes such as Schottky diodes may beplaced in anti-parallel with the MOSFET switches primarily for reverserecovery and in some cases rectification, rather than relying on thebody diodes of the MOSFETs for these functions.

As with the ac-dc converter circuit 801, in this embodiment pulse-widthmodulation-based control of the switches is performed by a localembedded controller within the dc-dc converter circuit 901 (not shown).Such controllers will be familiar to those skilled in the art.

FIG. 10

An embodiment of one of the second set of ac-dc converter circuits isshown in FIG. 10 for use in the electrical system of FIG. 7.

Those skilled in the art will recognise the ac-dc converter circuit 1001as a three-phase converter, having a first pair of terminals 1002 and1003 forming a dc interface and three half-bridge legs 1004, 1005, and1006 formed of two switches with a respective polyphase terminal 1007,1008, and 1009 connected therebetween.

Various examples have been described, each of which feature variouscombinations of features. It will be appreciated by those skilled in theart that, except where clearly mutually exclusive, any of the featuresmay be employed separately or in combination with any other features andthe invention extends to and includes all combinations andsub-combinations of one or more features described herein.

1. An electrical system for connecting a rotary electric machine to dcnetworks operating at different voltages V and W where V>W, the electricmachine having N≥2 independent phases each having a respective indexn=(1, . . . , N), the electrical system comprising: a first set of Nac-dc converter circuits connected in a modular multilevelconfiguration, each ac-dc converter circuit having a respective indexn=(1, . . . , N) and an ac interface for connection with a correspondingnth phase of the electric machine, and in which the modular multilevelconfiguration has P=N+1 dc outputs each having a respective index p=(1,. . . , P) wherein the potential difference between the pth output andthe (p+q)th output is qV/N, where q=(0, . . . , P−p), a set of N dc-dcconverter circuits each having a respective index n=(1, . . . , N) andbeing configured to convert dc power between a voltage V/N at a first dcinterface and a voltage W at a second dc interface, wherein, for all n,a first dc interface of the nth dc-dc converter circuit is connectedwith the p=nth and p=(n+1)th dc outputs of the modular multilevelconfiguration.
 2. The electrical system of claim 1, in which 0.5 W≤V/N≤2W.
 3. The electrical system of claim 2, in which 0.8 W≤V/N≤1.3 W.
 4. Theelectrical system of claim 1, in which N is even and the ([N/2]+1)th dcoutput of the modular multilevel configuration is connected to anelectrical ground.
 5. The electrical system of claim 1, in which W is540 volts.
 6. The electrical system of claim 1, in which V is from 1kilovolt to 10 kilovolts.
 7. The electrical system of claim 6, in whichV is from 1 kilovolt to 3 kilovolts.
 8. The electrical system of claim1, in which the first set of N ac-dc converter circuits arebidirectional ac-dc converter circuits.
 9. The electrical system ofclaim 8, in which the first set of N ac-dc converter circuits compriseH-bridges.
 10. The electrical system of claim 8, in which the set ofdc-dc converter circuits comprise phase-shifted full bridges.
 11. Theelectrical system of claim 1, further comprising: a second set of Nac-dc converter circuits each having a respective index n=(1, . . . ,N), wherein for all n, a dc interface of the nth one of the second setof ac-dc converter circuits is connected with a second dc interface ofthe nth dc-dc converter circuits; a second rotary electric machinehaving N independent phases each having a respective index n=(1, . . . ,N), wherein for all n, the nth phase of the second electric machine isconnected with an ac interface of the nth one of the second set of ac-dcconverter circuits.
 12. The electrical system of claim 1, furthercomprising: a second set of N ac-dc converter circuits each having arespective index n=(1, . . . , N), wherein for all n, a dc interface ofthe nth one of the second set of ac-dc converter circuits is connectedwith a second dc interface of the nth dc-dc converter circuits; a secondelectric machine having N polyphase winding sets, each having arespective index n=(1, . . . , N), wherein for all n, the nth windingset of the second electric machine is connected with an ac interface ofthe nth one of the second set of ac-dc converter circuits.
 13. Theelectrical system of claim 11, in which the second set of N ac-dcconverter circuits are bidirectional ac-dc converter circuits.
 14. Theelectrical system of claim 11, in which the second set of N ac-dcconverter circuits comprise H-bridges.
 15. A gas turbine engine having alow-pressure spool and a high-pressure spool, and further comprising theelectrical system of claim 11, in which the first rotary electricmachine is connected with the low-pressure spool and the second rotaryelectric machine is connected with the high-pressure spool.
 16. Anarrangement comprising: a first gas turbine engine having a first spool;a second gas turbine engine different from the first gas turbine engine,and having a second spool; and the electrical system of claim 11, inwhich the first rotary electric machine is connected with the firstspool and the second rotary electric machine is connected with thesecond spool.